The present invention relates to an apparatus for heating and controlling the process temperature of a tunnel pasteurizer, in particular for packaged food products.
As is well known, pasteurization of packaged food products is the heat treatment whereto are subjected some types of products already packaged in final containers, in order to improve their preservation over time.
The type of pasteurization referred to hereinafter is xe2x80x9clow temperature pasteurizationxe2x80x9d and with specific reference to food products constituted by drinks. This means that it takes place at a temperature lower than 90xc2x0 C. by means of hot water which is sprayed in a programmed manner onto the containers in order to modify their temperature according to a defined thermal cycle.
The apparatus whereby the pasteurization process is achieved is essentially constituted by a tunnel through which is treated the product already packaged in the containers (bottles, cans or other containers) which are made to advance in the tunnel by means of a conveyor.
From the thermal point of view, the tunnel is essentially subdivided into three areas: a first area (area 1) for pre-heating, where the temperature of the product is increased to a value beyond which the actual heat treatment takes place; a second area (area 2) for heat treatment; a third area (area 3) for cooling, where the product is brought back roughly to ambient temperature in order to prevent undesired fermentation phenomena.
Each of these three area is further subdivided into two or more parts (sub-areas) in order to: (a) avoid thermal shocks and have the opportunity to recover heat between the heating and the cooling areas, or (b) have available areas at different temperatures, variable according to determined logic criteria, in the portion of the tunnel where the actual heat treatment takes place.
In accordance with the prior art, the heating of the water that is sprayed onto the product (process water) takes place by means of a plurality of heat exchangers, whereof each is associated to a sub-area of the areas 1, 2, 3 into which the tunnel is subdivided.
FIG. 3 shows a tunnel pasteurizer obtained according to the prior art in question (see also patent EP 960 574).
Each of said exchangers is formed by a single primary loop, whose fluid can be, depending on requirements, saturated steam, superheated water, hot water, etcetera, and by a single secondary loop wherein the process water circulates. Also present is a metering valve for the primary circuit and a condensation drain if the primary fluid is steam.
The process water circulates in each sub-area, through a hydraulic loop connecting the collection tank, located below the sprayed product, with sprinklers positioned above the product. The secondary loop of each heat exchanger is connected with the hydraulic loop of each sub-area, described above. To each sub-area of the pasteurizer, whose process water requires appropriate heating, must therefore be associated a specific heat exchanger.
While this constructive solution meets the operating requirements of the system very well, it does nonetheless have some drawbacks.
These are, essentially:
A relatively great complexity of the system for controlling and adjusting temperatures and hydraulic lines;
The need to provide for the maintenance of a high number of heat exchangers and of the equally numerous high pressure metering valves;
The inertia of the adjusting and controlling organs which are subject to continuous disposition variations;
The poor efficiency of the heat exchangers which are often called to operate in transient states.
To solve these series of drawbacks, some manufacturers have adopted a centralized heating system, which provides for the adoption of a single heat exchanger for all areas subjected to temperature control. This exchanger provides for heating a mass of water present inside a tank, which is maintained at sufficiently high temperature, and which is mixed with the process water of the various areas, in order to increase its temperature, depending on the need of each. Thermal energy is thus distributed by means of masses of hot water which are added in the areas where a temperature increase is required.
FIG. 2 schematically shows a tunnel pasteurizer obtained in accordance with this second type of prior art (see for instance patent WO 95/22352). This technical solution, while providing some unquestionable advantages over its prior art, still exhibits some drawbacks.
In particular:
High quantities of water masses inside the pasteurizer which must be heated before starting the conveyor (water masses present both in the process vats and in the common tank) with evident high energy expenditure;
Inevitable loss, at the final shut down of the pasteurizer, of the energy expended for the preventive heating of the water mass contained in the common tank;
Need for high water flow rates between the common tank and the various process vats if the temperature of the water in the common tank decreases (transferred power is equal to the product between water volume and temperature);
The existence of a single heat exchanger which, during the critical process control phases, must provide sufficient thermal energy both for the rapid heating of the areas of the pasteurizer, and for restoring the temperature of the water mass present in the common tank. And this with the eventuality that it may not be possible to perform the pasteurization process correctly and/or completely, with the consequent possible spoiling of the product;
The presence, inside the pasteurizer, of a great quantity of water at high temperature concentrated in a single area (the common tank) can alter, due to the heat exchange with the surrounding environment, the thermal equilibrium of the areas subject to temperature control;
The constant mixing of the water masses at different temperatures in the different areas of the pasteurizer requires the constant restoration of operating temperatures, once again with great energy expenditure.
The essential aim of the present invention therefore is to overcome the aforementioned drawbacks, relating to current systems for heating and controlling the process temperatures in tunnel pasteurizers, making available an apparatus that allows:
considerably to limit the number of heat exchangers used and the fluid regulating sets in their primary loop;
to limit the frequency of the variations in heat power supplied by the heat exchangers;
to maintain the process waters always separated from area to the other of the pasteurizer;
to avoid the use of a common tank containing large quantities of water at high temperature;
to simplify the structure of the hydraulic system of the pasteurizer.
These aims and others besides are all achieved by the subject apparatus for heating and controlling the process temperature, whose main characteristics are indicated in the claims that follow.